This disclosure generally relates to systems and methods for wireless power transmission. In particular, this disclosure relates to wireless power transmission by means of resonant inductively coupled transmitters and receivers.
Resonant inductive coupling is the near-field wireless transmission of electrical energy between two coils that are tuned to resonate at the same frequency. Resonant transfer works by making a primary coil ring with an oscillating current, which generates an oscillating magnetic field. A secondary coil in proximity to the primary coil can pick up energy from the oscillating magnetic field. If the primary and secondary coils are resonant at a common frequency, significant power can be transmitted from the primary coil to the secondary coil over a range of a few times the coil diameters at reasonable efficiency.
A known resonant inductive coupling method requires both a resonant frequency match and an orientation match between the transmitter and receiver for significant power transmission to occur. That known method matches frequencies, but because the system has a constant transmitter/receiver relative position and orientation, it does not need to address orientation matching.
In the case of a mobile receiver, the orientation of the mobile receiver relative to the power transmitter can change. However, the mobile receiver, and synonymously the target object in which that receiver is incorporated, must align with the magnetic field line to efficiently receive wireless power. The fact that stationary transmitters produce the same magnetic field limits the spatial freedom of the target object. The target object can change position and orientation to some degree relative to the power transmitters, but the cost is power transmission efficiency.
In the wireless power industry, one strategy has been to place the transmitter on the same plane as the typical desk on which the devices sit. Each device has an internal receiving coil aligned with a magnetic field produced by the transmitter. This allows for efficient charging. However, once the consumer picks up the device to use it, the wireless power link breaks off and the device stops charging. In other words, the availability of this source of power is limited.
In addition, any wireless technology, whether it is data or power transmission, requires interference consideration. A known method of wireless power transmission uses different frequencies (e.g., 44, 62 and 77 kHz) for each of three receivers (e.g., windings) in a motor. Different frequencies were used so that both the magnetic field's frequency and orientation differentiation could be used to minimize crosstalk, or interference, between each of the three phases in the motor. The downside to this approach is that it occupies a frequency band which can result in interference with other surrounding wireless systems.
The problem of crosstalk between phases can be solved by using wider frequency differentiation in addition to orientation differentiation to produce a “double filter”. This is an effective approach, but the downside is that the transmitted power occupies a wide frequency band, which can result in interference with other surrounding wireless systems. Another way to view it is, each such system occupies a wide bandwidth, so that few systems can operate in a given volume of space.
An improved method for increasing the spatial freedom of the target object and reducing crosstalk between phases during resonant inductive coupling of power transmitters and receivers is desired.